In packaging food materials oriented polyester films “OPET” (Orientd PET) as pouches or thermoformed dual ovenable trays made from “CPET” (Crystalline PET) are nowadays very common. The use of such polyester containers need a cover, lid or seal which can both seal the container to prevent leakage of the contents during storage and as well protect the contents from external atmosphere. The top layer or seal should be easily peeled without tear from the container. The lid or seal normally comprises a flexible composite polymeric film consisting of a base polymer layer and a sealable layer of another polymer or a coating layer of another polymer. A strong seal with easy peeling properties is required at both low, e.g ambient and high temperatures. Also the heat seal layer should be transparent, haze free and optically clear.
However, heat sealing of PET containers is extremely difficult in comparison with other plastic containers, such as those from HDPE, PP and PVC, because of PET has high melting point of ˜260° C. and its reasonably fast crystallization rates at temperature between 100 to 240° C. This makes the heat sealing process difficult and if forcibly sealed at high temperatures, the material becomes brittle due to crystallization and polymer degradation. Thus it becomes important to inhibit crystallization of the polyester composition at the time of heat sealing in order to increase the strength of the heat seal.
Existing Knowledge:
Polyester resins for heat sealable films are known as early as from 1970s. U.S. Pat. No. 4,020,141 discloses the method of making heat-sealable, heat shrinkable, biaxially oriented polyester film wherein the polymer consisted primarily of PET with additional monomeric units selected from a second acid or a second glycol.
EP 0035835 deals with polymer film composites with a primary layer comprising an oriented layer of first linear polyester and a heat sealable secondary layer adherent to the primary layer which comprises essentially amorphous second linear polyester. It also states that the preferred polyester resin is an amorphous copolyester resin comprising PET and PIA (polyethylene isophthalate) or hexahydroterephthalic acid based co polyester with a preferred composition of PET:PIA as 80:20.
U.S. Pat. No. 4,765,999 discloses multilayer non oriented heat sealable films with a base substrate layer of PET or its co polymer and a heat sealable surface layer of copolyester consisting of DEG or PETG.
JP Patent 63202429 discloses a polyester shrink film with excellent heat sealing property. Here the polyester composition constituted PET as the major component and 3-50 mole % of neopentyl glycol as the glycol modifier.
JP Patent 1165627 discloses with a polyester sheet for heat sealing thermoformed objects and the polyester resin comprised PET with 1-15 parts by weight of a block co polyester of PBT and poly tetramethylene glycol and 2-20 parts by weight of PP resin.
EP 0379190 discloses a laminated polyester film with excellent heat sealing ability consisting of a base polyester layer and a heat sealing layer. The resin composition used in the heat-sealing layer comprises either glycol modified polyester or a dicarboxylic acid modified polyester which includes acids like adipic acid, azelaic acid, isophthalic acid etc. It also gives data on sealing energy, antiblocking property etc.
U.S. Pat. No. 5,484,632 discloses a non oriented heat sealing polyester film PET, EG and 7-15 mole % of CHDM, a glycol modifier.
WO 9619333 discloses a polyester based film with a substrate layer and a coating layer containing copolyester of aliphatic or aromatic dicarboxylic acid, which includes isophthalic acid, along with at least one polyalkylene glycol. The film is suitable for use as a lid for a container with strong heat sealing and easy peeling property.
U.S. Pat. No. 5,721,028 discloses heat-sealed closure for polyester container. The closure composed of a laminate of a heat seal layer comprising mainly a copolyester resin modified with aliphatic or aromatic dicarboxylic acids or glycol modified co polyesters.
U.S. Pat. No. 5,955,181 discloses an invention of a co-extruded heat-sealable polyester film wherein the heat sealable outer layer comprises a copolymer containing 5 to 95% by weight of ethylene 2,6-naphthalate units.
U.S. Pat. No. 5,972,445 discloses a multilayer polyester sheet with a base layer comprising a polyester resin and a sealing layer comprising a polyester resin with a Tg less than that of the base layer by at least 5° C. The sealing layer co polyesters are modified by NDC or IPA or DEG or CHDM or neopentyl glycol.
U.S. Patent Application 20020015835 discloses sealable polyester film comprising modified co polyesters with NDC, cycloaliphatic or aromatic diols and/or dicarboxylic acids.
U.S. Pat. No. 6,623,821 discloses a packaging film comprising heat sealable polyester film comprising a homopolymers or copolymer of PET and coated with a solvent bases heat seal coating of amorphous co polyester. The amorphous heat sealing copolyester has a composition consisting of PET and other diacid like succinic acid, isophthalic acid etc.
U.S. Pat. No. 6,902,802 discloses a heat-sealable polyester composition preferably containing a mixture of PET and PTT as homopolymers or copolymers, either as a physical blend or as a copolyester by in-situ preparation and claiming the composition's capability of heat-sealing at low temperatures with barrier properties and non-flavor scalping.
U.S. Patent application 20050061708 discloses a co-extruded heat sealable and peelable polyester film having high peeling resistance. The film comprises a base layer and a heat sealable top layer. The polymer of the heat sealable layer has a melting point of <190 to 230° C. and is a co polyester composed of units derived from aliphatic or aromatic dicarboxylic acids
U.S. Patent application 20050249906 discloses a process for the production of coated heat sealable polymeric film. The heat sealable layer consists of co-polyester with repeat units of one or more aliphatic or aromatic dicarboxylic acids whose concentration ranges from 40 to 70 mole %.